Modern electrical power distribution systems typically employ various types of apparatus for protection and automatic sensing of abnormal or undesired conditions in the system. Automatic reclosing relays are frequently used for automatic reclosure of AC circuit breakers after such breakers are opened because of overcurrent or due to other protective action. Such relays are adjustable or settable to typically provide a few reclosures at predetermined time intervals, so that in the event that the breaker does not remain closed after the first reclosure, additional reclosures will be made. The first reclosure is usually "instantaneous" (reclosure of the breaker with no intentional time delay). The first reclosure is often successful because many of the faults encountered in the systems are of a temporary nature, such as lightning flashovers and faults caused by tree limbs or birds.
If the preset number of breaker reclosures is unsuccessful, the timing drum or cam of the reclosing relay runs to its "lockout" position. Any further attempts of breaker reclosure must then be made by manual operation.
However, such apparatus is not generally reliable for handling continuous lower current ground fault conditions in the power distribution system. To demonstrate this problem, a typical breaker with protective relays could consist of a ground fault relay, such as a Type IAC53 time overcurrent relay, and a reclosing relay, such as a Type NLR11, both manufactured by General Electric Company of Philadelphia, Pa. The ground fault relay could be set for a time dial setting of 6 and a tap setting and current transformer setting to give a 150 ampere minimum pickup. The reclosing relay could have three reclosings set at instantaneous, 20 seconds, 40 seconds; and reset set at next close.
When a line or conductor of the distribution system breaks due to storms, accident or other occurrence, and falls to the ground, there may be a continuous fault current in the general range of 150 to 180 amperes. Under such conditions, the line to ground impedance is not usually zero, but about 40 ohms may be typical. Ideally, the ground fault should be immediately detected by the ground fault relay which in turn will open the circuit breaker in a time of about 90 seconds. The reclosing relay in sequence with the ground fault relay should operate through its preset reclosing intervals and cycle around to its lockout position leaving the circuit breaker in a locked open position. However, with the above stated relay settings and conditions with a 150 ampere continuous fault, the circuit breaker will not be taken to a locked open position. Due to incompatible time sequencing between the ground fault relay and the reclosing relay, the reclosing relay will continue to reset and never go to lock out. The breaker in return will continue to open and close never going to a locked open position. This will be better understood from further timing considerations that follow.